In Response To 'Re: Re: Comparing variables in a set & 3D plott...'
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Since I have no clue what code you executed to get that strange surface I have no idea what you are doing wrong.
But if you want plot the two half planes then
p := 5 - Abs[n - s];
Plot3D[p, {n, 0, 4}, {s, 0, 4}]
or
Plot3D[5 - Abs[n - s], {n, 0, 4}, {s, 0, 4}]